1. Field of the Invention
This invention relates to a magnetic disc device which protects bearings with magnetic seal using a electroconductive magnetic fluid and at the same time grounds static electricity generated on the disc etc.
2. Description of the Prior Art
Heretofore, there have been available such devices as shown in FIGS. 7 and 8.
In the drawings, Numeral 1 is a cylindrical housing, Numeral 2 is a shaft disposed at the center of the housing 1 and supported with bearing 3. "C" is a ring-shape seal member which consists of a ring shape of magnet 6 and ring-shape pole pieces 4, 5 fixed to the end surfaces thereof. The seal member "C" is fixed to the inner surface of an outer race 3b of the bearing 3 with a gap "g" provided in between the pole pieces 4, 5 and an inner race 3a of the bearing 3. The inner race 3a is fixed to the shaft 2, and the outer race 3b to the housing 1. The housing 1 and the shaft 2 are electrically conductive, and the pole pieces 4, 5 are of magnetic material and also electroconductive. Numeral 7 are electroconductive magnetic fluids drawn to the above described gap "g" by magnetic force. The electroconductive magnetic fluids are maintained in place by magnetic field formed in between the inner race 3a and the seal member "C" by the magnet 6. Numeral 8 is a disc inserted onto the shaft 2, Numeral 9 a magnetic head, Numeral 10 bearing balls, Numeral 11 a ball cage, and Numeral 12 a seal plate.
The operation is as follows:
Because the gap "g" between the seal member "C" and the inner race 3a is sealed by the electroconductive magnetic fluids 7 held in between the both, the sealed gap can prevent dust (grease mist) from entering the inner clean zone from the bearing 3.
Further, because the pole pieces 4, 5 and the inner race 3a are made electroconductive through the electroconductive fluids 7, and the pole pieces 4, 5 and the outer race 3b are kept electroconductive through the electroconductive bond fixing the both, the shaft 2 is made electrically conductive to the housing 1. As a result, static electricity generated on the disc 8 is grounded through the shaft 2, the inner race 3a, the pole pieces 4, 5 and the housing 1.
But, since the resistance of the above-mentioned electroconductive magnetic fluid is extremely high, causing a poor conductivity, there have been a need for a less resistant system.
For this reason, the above-mentioned prior art made the axial thickness of the pole pieces 4, 5 as large as possible, to have large contact areas between the electroconductive magnetic fluid 7 and the pole pieces 4, 5 as well as the electroconductive magnetic fluid 7 and the inner race 3a.
Nevertheless, since the space in the bearing 3 is extremely limited, the space imposes a certain limit to the thickness of the pole pieces 4, 5, thus resulting in a failure to satisfy the above requirement. Therefore, the prior art could not succeed in an effectual grounding of static electricity generated on the disc.
Further, there has been available the U.S. Pat. No. 4,604,229 similar to the above-mentioned prior art.
This prior art (U.S. Pat. No. 4,604,229) has a construction in which a seal member similar to the seal member shown in FIGS. 7, 8 is directly inserted on the shaft at a position removed from the bearing, to obtain both of a static electric ground effect and a magnetic seal effect by electroconductive magnetic fluid magnetically held in a gap with the shaft.
Therefore, the U.S. Pat. No. 4,604,229 has a problem essentially similar to the above-mentioned precedent prior art.
Furthermore, there has been available the U.S. Pat. No. 4,673,997, the seal device using a electroconductive magnetic fluid similar to the prior art.
This prior art (U.S. Pat. No. 4,673,997) has another construction in which a space section accomodating a electroconductive magnetic fluid working as bearing is formed in between the shaft and the housing pivoting the shaft. In front and back of the space section, ring-shaped pole pieces of ring magnet are disposed to undergo magnetic force. In a gap between the inner end of the ring-shaped pole pieces and the periphery of the shaft, electroconductive magnetic fluid is thus magnetically maintained to be made magnetic seal.
As a result, electroconductive magnetic fluid magnetically held in between the shaft and the ring-shaped pole pieces can be a magnetic seal, and at the same time a sufficient amount of the fluid accomodated in the space section can be a good electroconductor for grounding. However, not only a great amount of precious electroconductive fluid filled in the space section increases the cost of the device, but also an extremely complicated construction of the space section to hold the electro magnetic fluid within the magnetic seal at both ends also increases the cost.
Further there have been available such devices using only magnetic fluid rather than the electroconductive magnetic fluid, as disclosed in the SU No. 916-844, Japan Laid-Open Patent No. 74972/1986, and Japan Utility Model Laid-Open Publn. No. 98868/1987.
The former, the Soviet Union Patent prior art has such a construction as follows: A ring magnet is fixed to the shaft. To the both sides of the magnet, two ring pole pieces are fixed, and magnetic fluid is magnetically held in between the peripheral end of the two ring pole pieces and the inside of the magnetized housing. The latter Japanese patent or publication is a magnetic seal device in which magnetic fluid is held in the housing through a magnet fixed to the shaft and through pole pieces to function as magnetic seal. In other words, in all the prior art, magnetic fluid is magnetically held not directly to the outer periphery of the shaft, but indirectly through pole pieces at a position outwardly removed from the peripheral surface of the shaft. As seen from the construction, they only aim at sealing and cannot expect such a sufficient electroconductivity as to ground static electricity generated.
Furthermore, there has been available such a device as disclosed in Japan Laid-Open Patent No. 76053/1985 in which: To keep magnetic fluid in sliding contact to the shaft, two pole pieces are fixed to each other across a ring magnet. The inner ends of the pole pieces are bent to opposite directions to each other so as to face each other, thus making magnetic field to keep magnetic fluid thereby.
The above prior art only maintains a gap around the outer periphery of the shaft and bends the inner ends of two pole pieces, in order only to keep magnetic fluid on the non-magnetic shaft. And yet the seal by means of the magnetic fluid employs only one stage of sealing, thus resulting in a failure to achieve a sufficient seal effect.